Selective conversion of biomass-derived furfuryl alcohol into n-butyl levulinate over sulfonic acid functionalized TiO₂ nanotubes

Sulfonic acid functionalized titanate nanotubes were prepared by the sulphonation reaction of hydrothermally synthesized TiO₂ nanotubes (TNTs) using chlorosulfonic acid as the sulfating agent. The as-prepared catalysts were characterized by fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD) analysis, thermogravimetry-differential thermal gravimetry (TG-DTG) and X-ray photoelectron spectroscopy (XPS) techniques. The characterization results revealed that the catalysts retained the tubular structure of the TNTs and possessed a large number of active sulfonic acid sites. The catalytic performance of the catalysts for the synthesis of n-butyl levulinate was investigated via the alcoholysis of biomass-derived furfuryl alcohol under atmospheric pressure. The effects of the reaction factors such as the catalyst dosage, reaction time, and temperature on the alcoholysis of the furfuryl alcohol were systematically studied. Under mild conditions, about 79.9% yield of n-butyl levulinate was achieved. In addition, the catalysts showed a stable catalytic performance after four consecutive cycles. Furthermore, no leaching of the active species was observed during the hot filtration testing, which can be attributed to the covalently linked –SO₃H groups on the TNTs surface. In addition, the opened tubular nanostructure of the catalyst and the introduced strong Brønsted acid sites exhibited synergistic effects, which facilitated the selective conversion of the furfuryl alcohol to butyl levulinate.